Compiling CLIPBOARD for non-X environments

[masinter]

Originally posted by @waywardmonkeys in #211 (comment)

In the long run, I think that this needs to be a subr so that maiko can provide the correct implementation per platform. Running natively on macOS or Windows would need different code from running within X, as would running via WebAssembly in the browser.

Until this is done, though, shouldn't we at least make sure that the extra utilities to be used for various things we;re using shell to do -- xclip, convert ps to pdf, run different scripts